ated 14 people for gunshot wounds and others suffering from the effects of tear gas.

But General Yahya Saleh, the head of Yemen's security forces, denied that live ammunition had been used against protesters.

"We are using tear gas canisters we imported from countries that promote human rights. These are not not homemade weapons," Saleh said in an interview with Al Jazeera's Hashem Ahelbarra.

"You can go and film our water water cannon trucks. Everything within the law. One person was killed because of suffocation. We are sorry for that. we are not using live ammunition otherwise the death toll would have been very high."

Elsewhere, two protesters were reported shot dead as police broke up demonstrations in the southern province of Aden.

The latest crackdown comes as pro-democracy opposition groups and students escalate their campaign to remove Ali Abdullah Saleh, Yemen's president for 32 years, from power.

Anti-government protesters have been camped out for days at Tagheer Square demanding Saleh step down.

In Sanaa, witnesses said police and supporters of the ruling General People's Congress party attacked protesters occupying the square on Sunday with live gunfire and tear gas.

Several thousand people had gathered early in the day, setting up barricades in an effort to separate themselves from riot police.

Our correspondent said security forces along with government loyalists started opening fire on the protesters, beating them with batons, throwing rocks and also firing tear gas canisters.

"At least 70 people have been injured, 14 of whom had bullet wounds ... two of them in very critical condition," Ahelbarra said.

"I've seen more than 40 people being carried to the makeshift clinic set up by the protesters, suffering from the suffocating effect from the tear gas."

The United States, which considers Saleh a key regional ally, urged the government on Sunday "to show restraint and to respect the universal rights" of the people.

Streets to the square were blocked with tents earlier on Sunday, many flying flags and other symbols denoting the provinces of demonstrators who have come from around the country to join the opposition sit-in.